1. Field of the Invention
The present invention relates to a recording tape cartridge for rotatably accommodating a reel on which a recording tape is wound.
2. Description of the Related Art
In a conventionally known type of recording tape cartridge for a recording medium for use as an external storage device of a computer or the like, a single reel, on which a recording tape such as a magnetic tape is wound, is rotatably accommodated in the recording tape cartridge.
A magnetic tape is used, for example, for storing computer data. The tape cartridge is structured so as to prevent the magnetic tape, which may contain important information, from jamming or the like, and from being unwound unintentionally when not in use, such as during storage or the like.
The reel comprises a reel hub and upper and lower flanges, which are disposed at both ends of the reel hub. As shown in FIG. 29, a reel hub 500 and a lower flange 502 are integrally molded and, after the molding, an upper flange 504 is welded to the reel hub 500 to form a reel 506.
A magnetic tape can be wound on the reel hub 500, and width-directional end portions of the wound-on magnetic tape are retained by the upper and lower flanges 504 and 502.
As shown in FIGS. 29 and 30, the reel hub 500 is provided with a driven gear 508, which meshes with a driving gear for transmitting driving force from a drive-side driving device (not shown).
At an inner side of the driven gear 508 is disposed a metal plate 510 for magnetic attachment. The metal plate 510 is magnetically attracted by a magnet disposed at the drive-side driving device, thus preventing misalignment of the axis and keeping the meshing state of the driven gear 508 and the driving gear. However, because the reel hub 500 is provided with the driven gear 508, the metal plate 510, and the like, variations of pressure during the molding may affect the lower flange 502 molded integrally with the reel hub 500, reducing flatness of the lower flange 502, and this may be a cause of surface-runout in the drive. Further, because the flatness of the lower flange 502 is lowered, roundness of the reel hub 500 deteriorates.
The metal plate 510 is integrally molded with the reel hub 500 and the lower flange 502 inside a metal mold which molds the reel hub 500 and the lower flange 502. Three through-holes 512 for joining the metal plate 510 to the reel hub 500 are formed at regular intervals in the metal plate 510 on a concentric circle thereof. During molding, a molding material flows into the through-holes 512 to fix the metal plate 510 to the reel hub 500.
In the metal mold, a position corresponding to an inner circumferential portion of the reel hub 500 is a gate position. When the reel hub 500 and the lower flange 502 are molded, the molding material flows in a space defined by the metal mold from the position corresponding to the inner circumferential portion of the reel hub 500 towards positions corresponding to a distal portion of the reel hub 500, and in radial directions towards positions corresponding to an outer circumferential portion of the lower flange 502.
When the molding material flows into the through-holes 512 formed in the metal plate 510, air in the through-holes 512 causes friction that resists the molding material. Therefore, comparing with a case in which the molding material flows around the metal plate 510, the pressure is increased. This causes variations in the pressure in a cavity 516, and disturbs stress balance.
Because the stress balance is disturbed when the molding material is filled, the disturbed stress balance is manifested as residual stress after the molding, and causes deformation of the reel hub 500 and the lower flange 502 shown in FIG. 29. Therefore, there is a risk of lowering the accuracy of the reel hub 500 and the driven gear 508, and the flatness of the lower flange 502.
Further, as shown in FIG. 31, because within the through-hole 512 (shown by arrows) moves during hardening of the molding material, wrinkles 518 are produced. Particularly, if the wrinkles 518 produce differences in the level of the outer circumferential surface of the reel hub 500 (see FIG. 29), a winding state of the magnetic tape on the reel may be disturbed, and magnetic surfaces at the inner and outer circumferential sides of the magnetic tape may be scratched by the tape's edges.